“Dropwise”. Literally. NOT.

“With good stirring at -78 ºC under argon, a solution of n-butyl lithium in hexanes (2.5M, 37.5 mmol, 15 mL) was added dropwise over 20 mins.”

… this is what actually happened…

“Alright, the syringe is loaded and the needle inserted. Let’s first add a tiny amount of BuLi and see what happens. OK, no fire and no explosion. Cool, another couple of drops and ooops… over two mililiters in a single squirt. Rats. These damn dispoable syringes! Let’s take a quick step back here and pull down the shield, just in case. Hm… except from the dry ice going bananas beneath the acetone in the cooling bath, everything else seems fine. What the heck, just a little more and… *bing*… Aha, a new email! Oh, the latest results from the primary assay are in. Gotta check if my last compound made it. Where was I? Oh, the BuLi addition, right. Hm, when exactly did I start the addition? Five minutes ago? Err… let’s compensate a little here; some rapid addition over the next minutes should do it. Nice developing color by the way; a clear deep orange solution must mean we’re good. Back to the computer. Oh snap, my compound wasn’t tested yet. The reaction again. I’ve added more than half of the BuLi in what may have been around 12 minutes. It was pretty unevenful. The other guys are going on lunch now and I really wanna tag along. In goes the rest of the BuLi!”

For quenching, indeed… A couple of years ago I ran a number of FC acylations with 1.2 eq AlCl3. Anything but literal dropwise quenching (and LOTS of ice and massive stirring) gave a reaction mixture volcano. Don’t try this at home.

…and now the question…. why is so important the “slow addition” or “dropwise”. My experience with lithium chemistry (some) says that slow addition is only required when running lithiations under Barbier conditions, otherwise though exothermic reactions might take place, the fast kinetics avoids extreme overheating… what do you think?

I think it all depends on the time over which the supposed dropwise addition is said to take place. Under 5 minutes and with a decent total volume of reagent, I’d believe it was really drop by drop, because someone might actually have the patience… but under half a mL or over 5 minutes, unless there’s a syringe pump involved, it’s just like you describe.

I used to do the same, now this:
The BuLi is syringed into a separate flask which is connected by cannula to the reaction flask. The BuLi flask is cooled on an ice bath, then allowed to warm SLOWLY to room temperature. The gradual pressure increase pushes the BuLi dropwise[1].